The present invention is directed to communication systems employing a spreading sequence for spreading transmitted information and for despreading received information. Such communication systems are sometimes referred to as employing spread spectrum techniques. One example of such a communication system is a communication system employing Direct Sequence Spread Spectrum (DSSS) techniques. The present invention is particularly useful in synchronizing communications for effecting spreading and despreading in successive time slots in a Time Division Multiple Access (TDMA) communication system employing spread spectrum techniques, including DSSS techniques.
Spread spectrum signals are used in commercial and military communication systems because they enable reliable and secure exchange of information between parties in harsh communication environments (e.g., environments having multipath propagation, intentional and unintentional jamming or other problems) while providing efficient use of the radio spectrum. Spread spectrum communications distributes a transmitted signal over a wide range of frequencies so that the power spectral density of the transmitted signals is relatively low. Designated receivers then can despread a received spread spectrum signal because the structure of the spreading sequence is known. The despread signals are demodulated, decoded, and, if necessary, decrypted. Properly performed spreading facilitates coping with multipath propagation and jamming. The relatively low power spectral density of the transmitted spread spectrum signals decreases probability of signal detection and interception, reduces interference with other signals transmitted in the same frequency band, and allows code division multiplexing.
In a direct sequence spread spectrum (DSSS) communication system, achieving high security of communications requires employment of a complex-valued virtually infinite nonlinear nonrecurrent pseudonoise (NN PN) sequence for spreading transmitted data. Such a complex-valued NN PN sequence (hereinafter referred to as a “spreading sequence”) is often comprised of two independent, real-valued NN PN sequences. One of the sequences is an in-phase (I) component and the other sequence is a quadrature (Q) component. A generator of such a complex-valued spreading sequence is a very expensive device.
There are many situations, in which it is reasonable to use Time Division Multiple Access (TDMA) communications. TDMA means that a number of users share the same frequency band by assigning this band to each user for a short duration of time called a time slot. The time slots are grouped into frames. The frame structure repeats, so that a user can operate (transmit and receive information) within one or more assigned time slots in each frame. TDMA may be employed to increase traffic volume that can be handled by a communication system. Capacity of a TDMA system may be further increased, for example, when directional antennas are used. This is an implementation of Space Division Multiple Access (SDMA) that adds to capacity of a TDMA system and significantly simplifies communication protocols.
Combined use of spread spectrum transmissions, directional antennas, and TDMA provides very effective and efficient utilization of available spectrum and transmitter power. Use of such a combination of technologies also further decreases probabilities of detection and interception of the signal and radically improves antijamming capabilities of a communication system. There are, however, some problems related to TDMA spread spectrum communications.
One problem associated with TDMA spread spectrum communication systems is related to the time and reliability of synchronization. A virtually infinite NN PN spreading sequence is the only type of spreading sequence presently used for data spreading that has proven to be unexploitable by an interceptor. Such resistance to interception is due at least in part to a requirement that alignment (synchronization) of incoming and reference NN PN spreading sequences is absolutely necessary for proper functioning of a spread spectrum communication system. In a TDMA spread spectrum communication system, this alignment has to be performed during each time slot. Conventional methods for effecting the required alignment have to date required significant time and very expensive equipment to implement. This has proven so at least in part because the conventional methods use a section of the spreading sequence for establishing the required alignment.
It is preferred that synchronization (alignment) time should not exceed a small fraction of the slot length or duration. If too long a duration is required for establishing synchronization, an acceptable throughput of a TDMA spread spectrum communication system cannot be achieved. Further, synchronization equipment that is expensive and complex can make a communication system commercially noncompetitive. Virtually all attempts to reduce synchronization time and complexity of synchronization equipment using conventional methods, apparatus, and systems have so far lead to insufficient reliability of synchronization.
Another problem associated with TDMA spread spectrum communication systems is related to the length of guard intervals, which are necessary in each time slot. The reason for the first or leading guard interval in a time slot relates to clock uncertainties among various communicating stations in a communication system or network. Reference clocks of all stations that constitute a network or can potentially join a network are usually periodically (for example, with period of 1 second) synchronized by signals of the global positioning system (GPS). Despite such periodic synchronization, there is still time uncertainty among stations for many reasons. The largest component of the time uncertainty is commonly caused by differing frequency drifts of the various communication stations' reference oscillators. Because of the resulting time uncertainty among various stations, a transmitter cannot start transmitting immediately at the beginning of a time slot without risking a receiving station missing transmitted information. To avoid a loss of information transmitted during a time slot, a transmitting station should start transmitting (according to its own clock) only after the end of a first or leading guard interval that exceeds the maximum possible time uncertainty among clocks of the various stations. A corresponding receiver should start reception immediately at the beginning of the time slot (according to its own clock) when distance between transmitter and receiver is unknown. A corresponding receiver should start reception at a delayed time after the beginning of the time slot when distance between transmitter and receiver is known. The delay should be equal to propagation time (according to the receiver's own clock). Propagation time is, of course, related to the distance between transmitter and receiver.
A second or ending guard interval at the end of a time slot is necessary for accommodating signal propagation time and clock uncertainties among various communicating stations in a communication system or network. A transmitter should stop transmitting before the end of a time slot to take into account propagation time of a transmitted signal and time uncertainty among clocks of the various stations. If a transmitter transmits until the end of a time slot, a signal transmitted during a first time slot could reach a receiver at the beginning of the next succeeding time slot, or a signal transmitted at the end of a first time slot according to the clock of a transmitting station could be transmitted at the beginning of the next succeeding time slot according to the clock of a receiving station. When distance between transmitter and receiver is unknown, the second or ending guard interval should be established as sufficiently long to accommodate the maximum possible propagation time a transmitted signal may take to travel from a transmitter station to a receiver station. Knowledge of the distance between transmitter and receiver allows reduction of the second guard interval so long as the distance is not a maximum.
When the first (leading) and second (lagging) guard intervals are significant compared to the length of the time slot, throughput of the communication system is substantially reduced. Conversely, reduction of the guard intervals is important for improving throughput of a TDMA spread spectrum communication system.
Yet another problem experienced by TDMA spread spectrum communication systems is related to the complexity of the spreading equipment. Conventional systems providing duplex communications require using two generators of virtually infinite NN PN spreading sequences for each station (one for the transmitter part of a communication station and another one for its receiver part). As mentioned earlier herein, such generators are usually very expensive. The generators' costs may significantly influence the overall cost of communication system.
There is a need for a spread spectrum communication system, including a spread spectrum TDMA system, that can effect synchronization for duplex communications using only one spread sequence generator per communication station.
There is a need for a spread spectrum communication system, including a spread spectrum TDMA system, that can quickly establish synchronization for carrying out communication operations in each time slot in order to enhance throughput of the system.
There is a need for a spread spectrum communication system, including a spread spectrum TDMA system that can adaptively alter various operational parameters to accommodate various operational conditions.